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(12) United States PatentChristensen

(54) MINIATURE TUNABLE LASERSPECTROMETER FOR DETECTION OF ATRACE GAS

(71) Applicant: California Institute of Technology,Pasadena, CA (US)

(72) Inventor: Lance E. Christensen, Baldwin Park,CA (US)

(73) Assignee: CALIFORNIA INSTITUTE OFTECHNOLOGY, Pasadena, CA (US)

(*) Notice: Subject to any disclaimer, the term of thispatent is extended or adjusted under 35U.S.C. 154(b) by 0 days.

(21) Appl. No.: 14/162,578

(22) Filed: Jan. 23, 2014

(65) Prior Publication Data

US 2014/0204382 Al Jul. 24, 2014

Related U.S. Application Data

(60) Provisional application No. 61/755,832, filed on Jan.23, 2013.

(51) Int. Cl.GOIN 21/25 (2006.01)GOIN 21/39 (2006.01)

(Continued)

(52) U.S. Cl.CPC ........... GOIN 21/39 (2013.01); GOIN 21/031

(2013.01); GOIN2113504 (2013.01);

(Continued)

(58) Field of Classification SearchCPC .. GO1N 21/3504; GO1N 21/39; GO1N 21/031;

GO1N 2021/399; GO1J 3/42; GO1J 3/26

(Continued)

(io) Patent No.: US 9,671,332 B2(45) Date of Patent: Jun. 6, 2017

(56) References Cited

U.S. PATENT DOCUMENTS

4,035,083 A * 7/1977 Woodriff ................... GOIJ3/28250/225

5,177,566 A * 1/1993 Leuchs ................ GO1B 9/0207356/43

(Continued)

FOREIGN PATENT DOCUMENTS

WO WO 02/073757 A2 9/2002WO WO 2006/007446 A2 1/2006

(Continued)

OTHER PUBLICATIONS

Herman, et al., JPL Laser Hygrometer (JLH) for in-situ water vapormeasurements, Jet Propulsion Laboratory, California Institute ofTechnology, 2012, 1 page.

(Continued)

Primary Examiner Tarifur Chowdhury

Assistant Examiner Mohamed K Amara

(74) Attorney, Agent, or Firm Lewis Roca RothgerberChristie LLP

(57) ABSTRACT

An open-path laser spectrometer (OPLS) for measuring aconcentration of a trace gas, the OPLS including an open-path multi-pass analysis region including a first mirror, asecond mirror at a distance and orientation from the firstmirror, and a support structure for locating the mirrors, alaser coupled to the analysis region and configured to emitlight of a wavelength range and to enable a plurality ofreflections of the emitted light between the mirrors, adetector coupled to the analysis region and configured todetect a portion of the emitted light impinging on thedetector and to generate a corresponding signal, and anelectronic system coupled to the laser and the detector, andconfigured to adjust the wavelength range of the emittedlight from the laser based on the generated signal, and to

(Continued)

502 Provide an open-path laser spectrometercomprising an analysis region having first andsecond mirrors, a laser, a detector, and an

electronic system

504Expose the analysis region of the open-pathspectrometer to the ambient atmosphere

506 ̀ Detect a characteristic of the impinging lightVI and generate a corresponding signal

505 ̀ I Adjust the wavelength range of the emittedy light from the laser based on the generated

signal

510 _~ Measure a concentration of the trace gas basedon the generated signal.

END

S

500

https://ntrs.nasa.gov/search.jsp?R=20170006147 2018-08-30T08:12:08+00:00Z

US 9,671,332 B2Page 2

measure the concentration of the trace gas based on thegenerated signal.

20 Claims, 6 Drawing Sheets

(51) Int. Cl.GOIN 21/3504 (2014.01)GOIN 21/03 (2006.01)

(52) U.S. Cl.CPC ................ GOIN 202113513 (2013.01); COIN

20211399 (2013.01); COIN 220110221(2013.01)

(58) Field of Classification SearchUSPC ............. 356/432, 440, 73, 454; 250/339.13;

73/23.2, 23.37See application file for complete search history.

(56) References Cited

U.S. PATENT DOCUMENTS

5,764,678 A * 6/1998 Tada ......................... HO IS 3/03372/107

6,570,159 132 5/2003 Ankerhold6,643,018 132 11/2003 Chavanne6,713,754 131* 3/2004 Mueller ................. G03B 27/73

250/2266,753,960 131* 6/2004 Polynkin ................... GOIJ3/02

356/3307,174,099 131* 2/2007 Chinn .................. H04B 10/564

372/29.017,830,926 131* 11/2010 Kim ....................... B82Y 20/00

372/202003/0043378 Al 3/2003 DiDomenico et al.2003/0095736 Al* 5/2003 Kish, Jr . ................ B82Y 20/00

385/142003/0231665 Al* 12/2003 Ichino ..................... HO IS 5/042

372/322004/0151438 Al* 8/2004 Ferguson ............... G02B 6/264

385/782006/0039009 Al* 2/2006 Kiesel ................... GOIJ9/0246

356/5192006/0237657 Al* 10/2006 Gamiles et al . .............. 250/3722007/0246653 Al* 10/2007 Zhou .......................... 250/339.12007/0273882 Al* 11/2007 Smith ........................... 356/4372008/0062404 Al* 3/2008 Kawano .................... GO1J 5/08

356/732011/0051772 Al * 3/2011 Fukuda .................. B82Y 20/00

372/50.112011/0228275 Al * 9/2011 Xia ........................ GOIN 21/77

356/437

2011/0251800 Al* 10/2011 Wilkins .......................... 702/242011/0285998 Al* 11/2011 Hara et al ..................... 356/4372013/0130400 Al* 5/2013 Harbers .................... GOIJ3/28

436/1712013/0211218 Al* 8/2013 Suzuki ................. A61B 5/1455

600/32820 13/03 173 72 Al* 11/2013 Eberle ..................... GOIL 1/246

60OA782013/0321802 Al* 12/2013 Imura ................... GO 1J 3/0297

356/3062014/0085632 Al* 3/2014 Preston ................. GO 1J 3/0205

356/326

FOREIGN PATENT DOCUMENTS

WO WO 2010/053612 Al 5/2010WO WO 2010/070147 Al 6/2010WO W020 13 1 19320 * 8/2013 ............. GOIN 21/00

OTHER PUBLICATIONS

Park et al., Development of the detection system of methane leakageusing 3.2 µm mid-infrared LED and PD, 4 pages.Khan, et al., Low power greenhouse gas sensors for unmannedaerial vehicles, Remote Sensing, May 9, 2012, pp. 1355-1368, vol.4.Tarsitano, et al., Multilaser herriott cell for planetary tunable laserspectrometers, Optical Society of America, Oct. 2007, pp. 6923-6935, vol. 46, No. 28.International Search Report, dated Apr. 28, 2014, and mailed May1, 2014, corresponding to PCT/US2014/012766, 3 pages.Written Opinion of the International Searching Authority dated Apr.28, 2014, and mailed May 1, 2014, corresponding to PCT/US2014/012766, 3 pages.Herriott, Donald R., et al. "Folded Optical Delay Lines." AppliedOptics. vol. 4, No. 8. Aug. 1965: 883-891.EPO Search Report and Opinion dated Oct. 28, 2016 in correspond-ing European Patent Application EP14743786 (14 pages).Christensen et al., "Aircraft and balloon in situ measurements ofmethane and hydrochloric acid using interband cascade lasers",Applied Optics, 2007, 46(7), pp. 1132-1138.Christensen et al., "Thermoelectrically cooled interband cascadelaser for field measurements", Optical Engineering, 2010, 49(11),pp. 111119-1-111119-6.Massie et al., "Design of a portable optical sensor for methane gasdetection", Sensors and Actuators B: Chemical: International Jour-nal Devoted to Research and Development of Physical and Chemi-cal Transducers, Elsevier BV, NL, 2006, 113(2), pp. 830-836(Abstract).Sonnenfroh et al., "Interband cascade laser-based sensor for ambi-ent C114", Optical Engineering, 2010, 49(11), pp. 111118-1-111118-10.

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US 9,671,332 B2

MINIATURE TUNABLE LASERSPECTROMETER FOR DETECTION OF A

TRACE GAS

CROSS-REFERENCE TO RELATEDAPPLICATION(S)

This application claims priority to and the benefit of U.S.Provisional Application No. 61/755,832, filed Jan. 23, 2013,the entire content of which is incorporated herein by refer-ence.

STATEMENT REGARDING FEDERALLYSPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made in performanceof work under a NASA contract, and is subject to theprovisions of Public Law 96-517 (35 U.S.C. 202) in whichthe contractor has elected to retain title.

FIELD

The present invention is directed to a spectrometer, moreparticularly, to a tunable laser spectrometer for detecting atrace gas, and methods of detecting the same.

BACKGROUND

2while maintaining the chamber at nearly constant tempera-ture and pressure, which may be below ambient temperatureand pressure. This adds further complexity, size, and weightto the laser spectrometer, which drive up its power usage and

5 cost.Thus, what is desired is a low-cost, low-power, light-

weight, portable, laser spectrometer for detecting and/ormeasuring the concentration of trace gases, and a method ofusing the same to detect trace gases.

10

SUMMARY

15

20

25

A known and promising technique for measuring molecu-lar pollutants is laser spectroscopy. The technique, which 30uses a tunable and narrow linewidth laser light source, offerssensitive and selective detection of trace gases in the infra-red (IR) spectral region.

Laser spectrometers use radiation passing through a vol-ume of gas to detect a trace gas. As the radiation passes 35through the gas, some of its energy is absorbed by the gas atcertain wavelengths. The range of wavelengths at which atrace gas exhibits characteristic absorption depends on theproperties of the trace gas. For example, methane (CH,)strongly absorbs wavelengths of about 3.2 µm to about 3.5 40µm, while carbon monoxide absorbs wavelengths fromabout 4.2 µm to about 4.5 µm. The level of energy absorptionat the absorption wavelengths may be used to determine theconcentration of a trace gas.

Recent developments in IR laser light sources radiating in 45the 3µm to 10 µm wavelength range show great promise asnearly all molecules of trace gasses have characteristicabsorption bands within this region.

Laser spectrometers may be used to detect a plethora ofgasses including hydrocarbons, water vapor, and even cal- 50cium fluoride. Laser spectrometers are conventionally usedin wastewater treatment facilities, refineries, gas turbines,chemical plants, mines, gas distribution lines, and otherlocations where flammable or combustible gasses may exist,as well as in atmospheric research. 55

However, current solutions have many shortcomings.Conventional laser spectrometers often use, for example,Quantum cascade (QC) lasers, which suffer from low con-version efficiency (and, thus, have high power consumption)and require heavy and expensive cooling mechanisms. Fur- 60thermore, commercial laser spectrometers that use lower-power consuming diode lasers often operate at near-IRwavelength ranges, where trace gas molecules may exhibitlower absorbance as compared to the IR range. Additionally,conventional solutions employ complex closed optical 65chambers (e.g., closed gas cells) with sophisticated mecha-nisms that sample and drive ambient air into the chamber

Aspects of embodiments of the present invention aredirected to a low-power, small and portable, tunable laserspectrometer for detection of a trace gas, such as methane,and a method of detecting the trace gas using the same.

Aspects of embodiments of the present invention aredirected to providing an open-path laser spectrometer(OPLS) having an open optical path region (e.g., an open-path analysis region) exposed to the ambient atmosphere,and not requiring temperature and/or pressure control.

According to an embodiment of the present invention,there is provided an open-path laser spectrometer for mea-suring a concentration of a trace gas, the open-path laserspectrometer including: an open-path multi-pass analysisregion including a first mirror, a second mirror at a distanceand orientation from the first mirror, and a support structurefor locating the first and second mirrors; a laser coupled tothe open-path multi-pass analysis region and configured toemit light of a wavelength range and to enable a plurality ofreflections of the emitted light between the first and secondmirrors; a detector coupled to the open-path multi-passanalysis region and configured to detect a portion of theemitted light impinging on the detector and to generate asignal corresponding to a characteristic of the detectedportion of the emitted light; and an electronic systemcoupled to the laser and the detector, and configured toadjust the wavelength range of the emitted light from thelaser based on the generated signal, and to measure theconcentration of the trace gas based on the generated signal.The trace gas may exhibit a resonant frequency response

in the wavelength range of the emitted light, and other gasesin the atmosphere do not exhibit resonant frequencyresponses in the wavelength range. The trace gas may bemethane.The laser may include a semiconductor laser diode oper-

ating in an infrared range. The wavelength range may befrom about 3.2 µm to about 3.5 µm.The open-path multi-pass analysis region may be exposed

to the ambient atmosphere. For example, an optical path inthe open-path multi-pass analysis region may be at nearambient temperature and at near ambient pressure.The open-path multi-pass analysis region may include a

Herriott cell, wherein the first and second mirrors may beopposing concave mirrors, and wherein the first mirrorincludes a first hole configured to allow light to enter and/orexit the open-path multi-pass analysis region.The first hole may be configured to allow the emitted light

to enter the open-path multi-pass analysis region, andwherein the second mirror includes a second hole configuredto allow the reflected light to exit the open-path multi-passanalysis region.The open-path laser spectrometer may be configured to

utilize a direct laser absorption and/or 2f modulation/de-modulation spectrometry techniques.The electronic system may include a global positioning

system (GPS) configured to track a location of the open-path

US 9,671,332 B2

3laser spectrometer and to synchronize the location with thegenerated signal of the detector.The electronic system may further include a wireless

transceiver configured to enable communication betweenthe open-path laser spectrometer and an external device.The open-path laser spectrometer may have a trace-gas

detection sensitivity of about 10 parts per billion (ppb) in 1second.The open-path laser spectrometer may be portable and

hand-held.The distance between the first and second mirrors may be

between about 8 cm and about 20 cm and a total optical pathlength of the open-path multi-pass analysis region may bemore than 4 m.The open-path laser spectrometer may further include a

thermoelectric cooler (TEC) configured to control a tem-perature of an emission source of the laser, wherein theelectronic system is configured to adjust the wavelengthrange of the emitted light from the laser by detecting awavelength shift in a spectrum of the generated signal, andby signaling the TEC to control the temperature of theemission source of the laser.

According to an embodiment of the present inventionthere is provided a method for measuring a concentration ofa trace gas including: providing an open-path laser spec-trometer including: an analysis region including a firstmirror and a second mirror at a distance from the first mirror;a laser; a detector; and an electronic system, wherein thelaser is configured to emit light of a wavelength rangetoward the second mirror, the emitted light reflecting off ofthe first and second mirrors a plurality of times beforeimpinging on the detector; exposing the analysis region ofthe open-path laser spectrometer to the ambient atmosphere;detecting, by the detector, a characteristic of the impinginglight, and generating, by the detector, a signal correspondingthe detected characteristic of the impinging light; adjusting,by the electronic system, the wavelength range of theemitted light from the laser based on the generated signal;and measuring, by the electronic system, a concentration ofthe trace gas based on the generated signal.

Exposing the analysis region may include exposing anoptical path in a region between the first and second mirrorsto an ambient temperature and an ambient pressure.The trace gas exhibits a resonant frequency response in

the wavelength range of the emitted light, and other gases inthe atmosphere do not exhibit resonant frequency responsesin the wavelength range.The open-path laser spectrometer may further include a

thermoelectric cooler (TEC) configured to control a tem-perature of an emission source of the laser, wherein adjust-ing the wavelength range of the emitted light from the laserincludes: detecting, by the electronic system, a wavelengthshift in a spectrum of the generated signal, and signaling, bythe electronic system, the TEC to control the temperature ofthe emission source of the laser.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the presentinvention, reference is now made to the accompanyingdrawings, in which like elements are referenced with likenumerals. These drawings should not be construed as lim-iting the present invention, but are intended to be illustrativeonly.

FIG. 1 is a block diagram illustrating an open-path laserspectrometer (OPLS) for detecting a trace gas utilizing an

4open-path analysis region, according to an illustrativeembodiment of the present invention.FIG. 2 is a schematic diagram illustrating a perspective

view of the OPLS of FIG. 1, according to an illustrative5 embodiment of the present invention.

FIGS. 3A-3E are schematic diagrams illustrating thelaser, analysis region, and the concave lenses of the OPLS ofFIG. 1, according to illustrative embodiments of the presentinvention.

10 FIG. 4 is a block diagram illustrating the electrical systemof the OPLS of FIG. 1, according to an illustrative embodi-ment of the present invention.FIG. 5 is a flow diagram illustrating a process for detec-

tion of a trace gas, according to an illustrative embodiment15 of the present invention.

DETAILED DESCRIPTION

The detailed description set forth below in connection20 with the appended drawings is intended as a description of

illustrative embodiments of a system and method for detec-tion of a trace gas in accordance with the present invention,and is not intended to represent the only forms in which thepresent invention may be implemented or utilized. The

25 description sets forth the features of the present invention inconnection with the illustrated embodiments. It is to beunderstood, however, that the same or equivalent functionsand structures may be accomplished by different embodi-ments that are also intended to be encompassed within the

30 spirit and scope of the present invention. As denoted else-where herein, like element numbers are intended to indicatelike elements or features.The present invention relates to a system and method for

detection of a trace gas utilizing a compact, low-cost,35 low-power, tunable laser spectrometer having an open-path

multi-pass analysis region (e.g., an open-path multi-passoptical cell). The laser light source may be a low-powersemiconductor laser diode radiating in a narrowband of theinfrared range of the electromagnetic spectrum, where the

40 trace gas has characteristic absorption (e.g., about 3.2µn toabout 3.5 µm wavelength range for methane detection). In anembodiment, the open-path multi-pass analysis region maybe exposed to the ambient atmosphere, and, for example,may not be enclosed in a walled housing. The open-path

45 tunable laser spectrometer may be compact and lightweight,while exhibiting high detection sensitivity in the presence ofmechanical disturbances and temperature variations.FIG. 1 is a block diagram illustrating an open-path laser

spectrometer (OPLS) 100 for detecting a trace gas utilizing5o an open-path analysis region (e.g., open-path optical cell)

106, according to an illustrative embodiment of the presentinvention. In an embodiment, the OPLS 100 includes anelectronic system 102, a laser 104, which includes a ther-moelectric cooler (TEC)105, an analysis region 106 includ-

55 ing first and second mirrors 108/109, and a detector 110.According to an embodiment, the electronic system 102

may control the laser 104 by, for example, driving/excitingthe laser 104 and/or controlling the laser temperature andmay control the detector 110 and process the signal gener-

6o ated by the detector 110 to determine the concentration of atrace gas (e.g., methane) 112 in an atmosphere 114. Theelectronic system 102 may further transmit the collectedinformation from the detector 110 and/or the processedinformation to an external device.

65 In an embodiment, the laser 104 includes a light emissionsource, such as a semiconductor laser (e.g., a semiconductorlaser diode, a vertical-cavity surface-emitting laser (VC-

US 9,671,332 B2

5SEL), interband cascade (IC) lasers, and/or the like), emit-ting light of a narrowband within a wavelength range inwhich the trace gas 112 exhibits a relative high light absorp-tion as compared to other gases in the atmosphere 114. Thus,the wavelength range is chosen such that the frequencyresponse of the trace gas 112 of interest exhibits a resonance,which is distinguished from the frequency response of othergas molecules in the surrounding atmosphere 114. Forexample, if the trace gas 112 is methane, the wavelengthrange may be from about 3.1 µm to about 3.6 µm and thenarrowband of emission may be centered at a wavelengthbetween about 3.25 µm and about 3.38 µm (e.g., about 3.27µm). The laser 104 may be tunable and, in the example ofmethane detection, may have a spectral tuning range ofabout 0.6 cm-' (e.g., from about 3057.4 cm-' to about 3058cm i, where methane absorbs strongly but other molecules,such as water and carbon dioxide, do not).

According to an embodiment, the laser 104 includes aTEC 105 for temperature control to maintain (or improve)light emission output levels and wavelength integrity. Theelectronic system 102 tunes the laser 104 to near an absorp-tion line of the trace gas 112 by controlling the injected DCcurrent and by driving the TEC 105 to regulate the tem-perature of the light emission source.

In an embodiment, the drive current of the laser 104 maybe modulated by the electronic system 102 in order toimprove the absorption sensitivity and accuracy of the OPLS100. The electronic system 102 may introduce a sinusoidalmodulation current having a frequency f (e.g., 10 MHz) ontop of the DC drive current, which may shift the detectionband to a high-frequency region where the 1/f laser noise isreduced (e.g., minimized). In some examples, the frequencyf may be in the kHz, MHz, or even GHz ranges dependingon, for example, the absorption linewidth of the trace gas112, the detection bandwidth of the detector 110, and/or thedesired detection sensitivity of the OPLS 100.The light emission source may consume little power, for

example, a semiconductor laser diode may operate at lessthan 200 mA of current and less than 2.5 V voltage, thusenabling the open-path laser spectrometer (OPLS)100 to bepowered by an off-the-shelf battery, such as a AA or AAAbattery, which may allow for better portability of the OPLS100.The laser 104 may further include a mechanical housing

with optics configured to direct the laser light (e.g., the lightbeam) into the analysis region 106.

In an embodiment, the analysis region 106 is structured asan open-path multi-pass absorption cell, such as a Herriottcell, and includes a support structure for maintaining (e.g.,locating) the first and second mirrors 108/109 a distance Dapart. The distance D may be between about 8 cm to about20 cm (e.g., D may be about 15.4 cm). The support structuremay include two mirror holders and two or more connectingrods. In an example, the open-path multi-pass absorptioncell includes more than two mirrors (e.g., three mirrors, asin a White cell).

According to an embodiment, the analysis region 106 isnot encased in an enclosure and the analysis region 106 hasfree access to the ambient atmosphere. Thus, the analysisregion 106 and the volume between the first and secondmirrors 108/109 are at (e.g., are exposed to) the ambienttemperature and pressure. The open-path structure of theanalysis region 106 obviates the need for a pump, whichmay have been a necessary component in some conventionallaser spectrometers.The first and second mirrors 108/109 may be opposing

concave mirrors (e.g., facing spherical confocal mirrors). In

Tone example, the opposing mirrors 108 may have a diameterbetween about 1 cm to about 7.5 cm (or, e.g., between about2 cm to about 5 cm, such as about 2.5 cm), and a radius ofcurvature between 1 cm to about 70 cm (or, e.g., between

5 about 10 cm to about 50 cm, such as about 34 cm). In anembodiment, the opposing mirrors 108/109 include a sub-strate material having a low (e.g., very low) coefficient ofthermal expansion (e.g., having a thermal expansion coef-ficient lower than borosilicate glass), such as a lithium

io aluminosilicate glass-ceramic material, which allow themirrors 108/109 to retain their figures in the presence ofambient temperature changes. However, in an example,weight considerations may trump temperature sensitivityconcerns and the substrate material may be chosen from a

15 group of lightweight materials such as Aluminum.The mirrors 108/109 may include a coating of a reflective

material, such as gold, silver, aluminum, polished metal,and/or the like. The reflective coating may exhibit greaterthan 95% reflection at wavelength of about 3.3 µm. The

20 coating may also be reflective in the visible range of the lightwavelength spectrum to allow for easy alignment.In an embodiment, a hole may be created in one or more

of the mirrors 108/109 to enable the emitted light beam fromthe laser 104 to enter and exit the analysis region 106. In one

25 example, the light beam enters the analysis region 106 fromone side and exits from another side. Alternatively, the lightbeam may enter and exit the analysis region 106 from a sameside. The hole may be created by, for example, machining aphysical hole through the one or more mirrors 108/109, or

3o by removing a portion of the reflective coating on the one ormore mirrors 108/109 in the case of mirror substrate mate-rials that are transparent to the laser wavelength range.The open-path analysis region 106 (e.g., the open-path

multi-pass absorption cell) may be configured to allow35 multiple reflections (e.g., traversals) of the emitted light

beam between the first and second mirrors 108/109 beforefinally impinging on the detector 110, thus offering aneffective optical pathlength far greater than the distance D.The number of traversals, and thus, the effective optical

40 pathlength, may be controlled by adjusting the separationdistance D between the first and second mirrors 108/109and/or the diameter of the opposing mirrors 108/109. In oneexample, the open-path analysis region 106 permits 27traversals for a total optical pathlength of more than 4 m. As

45 the optical pathlength increases, there is greater opportunityfor the molecules of the trace gas 112 to interact with (andabsorb a portion of the energy of) the reflecting light beamas it traverses the open-path cell, thus detection sensitivitymay be improved (e.g., increased).

50 An embodiment in which the analysis region 106 has onlytwo mirrors that are spaced close to one another may be lesssusceptible to mechanical disturbances, and thus, permithigher detection sensitivity and portability, than an embodi-ment in which more than two mirrors are used or in which

55 the mirrors are spaced further apart.The detector 110 may include a photodetector (e.g., a

semiconductor detector or a photovoltaic photodetector),which may receive the reflected light beam through a hole inthe second mirror 109, which is opposite from the laser 104,

60 or through a same or different hole in the first mirror 108,which is adjacent to the laser 104. The photodetector may besmall, e.g., about 1 mm in size, and may be integrated witha lens (e.g., a hemispherical or hyperhemispheric lens)and/or optical filters for improved (e.g., increased) accep-

65 tance angle and saturation level. When a reflected light beamimpinges on the photodetector, the detector 110 generates asignal corresponding to a characteristic of the impinging

US 9,671,332 B2

7light (e.g., generates a current signal proportional to theintensity of the impinging light), which the detector 110transmits to the electronic system 102 for further processing.The detector 110 may include a preamplifier, such as atransimpedance preamplifier having an adjustable cut-offfrequency, for converting and amplifying the generatedsignal (e.g., converting the generated current signal to anamplified voltage signal) before transmitting it to the elec-tronic system 102. Keeping the photodetector and the pre-amplifier close together may improve (e.g., increase) thesignal-to-noise ratio (SNR) of the detector 110.

According to an embodiment, the detector 110 employs adirect absorption and/or a modulated detection scheme, suchas 2f, 4f, and/or the like, where f represents a modulationfrequency at the laser 104. In an embodiment, the preampmay generate, and subsequently transmit to the electronicsystem 102, one or more signals representing a modulatedsignal (e.g., a 2f signal) and/or a non-modulated signal(when direct absorption is employed).

In an embodiment, the OPLS 100 further includes a powerregulator for inputting power from a direct-current (DC)power source, such as a battery, and generating one or moreoutput voltages corresponding to the operating voltages ofthe various components of the OPLS 100. The powerregulator may be physically separated (e.g., isolated) fromthe electronic system 102 and the detector 110 to reduce(e.g., minimize) noise and improve (e.g., increase) thedetection sensitivity of the OPLS 100.

Accordingly, in an embodiment of the present invention,the OPLS 100 is compact (e.g., less than 18 cm in length),lightweight (e.g., weighing less than 300 gr), has low powerconsumption (e.g., less than 2 W), and is able to achieve ahigh detection sensitivity (e.g., about 10 ppb per second),while exhibiting mechanical robustness and low suscepti-bility to temperature variation. In some embodiments, theOPLS 100 may be configured as a hand-held spectrometer,which may be carried by a human user, and/or may beconfigured to mount to a vehicle, such as an unmanned aerialvehicle (UAV) platform.

While the numerical examples provided above were pri-marily related to detection of methane gas, by changing thecenter wavelength of the laser 104, and reconfiguring theanalysis region 106 by, for example, readjusting the distanceD between the opposing mirrors 108/109, modifying thediameter of mirrors 108/109, and/or modifying the coatingmaterial of the mirrors 108/109, the OPLS 100 may beadapted to detect a different trace gas, such as carbon dioxide(COZ) or water vapor (H20) in the atmosphere 114.

FIG. 2 is a schematic diagram illustrating a perspectiveview of the OPLS 100a of FIG. 1, according to an illustra-tive embodiment of the present invention.

According to an embodiment, the OPLS 100a includes amicrocontroller board 202, which includes an implementa-tion of the electronic system 102, a laser housing 204 forhousing the light emission source of the laser 104, opticalelements 205 for directing the light beam from the lightemission source toward the first and second mirrors 108a/109a, a mounting board 206 for coupling the analysis region106a to the laser housing 204, connecting rods for couplingthe first and second mirrors 108a1109a together and to themounting board 206, and a connecting scaffold 210 forcoupling the microcontroller board 202 to the mountingboard 206 and the analysis region 106a. According to anembodiment, the laser housing 204 is coupled to a firstmirror 108a and the detector 110a is coupled to a secondmirror 109a. The open-path volume between the first and

8second mirrors 109a is not encapsulated in a housing (e.g.,is uncovered) and is freely exposed to the ambient atmo-sphere 114.In an embodiment, the optical elements 205 are config-

5 ured to direct the light beam emanating from the laser 104athrough a hole formed in the first mirror 108a and toward thesecond mirror 109b. In another embodiment, the light emis-sion source of the laser 104 may be coupled to (e.g., directlycoupled to) the first mirror 108a in a manner such that no

io optical elements 205 are needed to direct its light beamtoward the second mirror 109a. Such an embodiment, whichdoes not utilize the optical elements 205, may be morerobust and less susceptible to mechanical disturbances thanan embodiment utilizing the optical elements 205 for direct-

15 ing the path of the laser light beam.FIGS. 3A-3E are schematic diagrams illustrating the

laser, analysis region, and the concave lenses of the OPLS ofFIG. 1, according to illustrative embodiments of the presentinvention.

20 FIGS. 3A, 313, and 3C are top, side, and front views,respectively, of the laser housing 204, the optical elements205, the mounting board 206, and the connecting rods 208,according to an embodiment of the present invention. Theoptical elements 205 may include, for example, an astig-

25 matic collimating optic with adjustable focus and an about90° turning mirror for directing the laser beam into theanalysis region 106a.FIGS. 3D and 3E are side and front views, respectively,

of the first mirror 108a, according to an embodiment of the30 present invention. In an embodiment, the first mirror 108a

includes a hole 212, which allows the light beam from thelaser 104 to pass through the first mirror 108a and toward thesecond mirror 109a. The first mirror 108a may have a radiusof curvature between 10 cm and 50 cm. In one example, the

35 hole 212 may be positioned 8.9 mm from the center of thefirst mirror 108a and have an average diameter betweenabout 2.7 mm to about 3.2 mm. The second mirror 109a mayinclude a similar hole for allowing the reflected laser beamto pass through the second mirror 109a and impinge on the

4o detector 110a.FIG. 4 is a block diagram illustrating the electronic

system 102 of the OPLS 100, according to an illustrativeembodiment of the present invention. According to anembodiment, the electronic system 102 includes a processor

45 (e.g., central processing unit (CPU)) 400, one or moreanalog-to-digital converters (A/Ds) 402, a laser driver 404,a global positioning system (GPS) 406, ancillary sensors408, a transceiver 410, and an input-output (I/O) port 412.The A/D 402 coupled to the detector 110 may digitize the

50 signal from the detector 110 for processing (e.g., real-timeprocessing) by the processor 400.The processor 400 utilizes the digitized signal from the

detector 110 to determine the concentration of the trace gas.In an example in which the laser drive signal is modulated

55 at a frequency f, the processor 400 may demodulate thedigitized signal at 2f, 4f, and/or 6f by utilizing, for example,a lock-in amplifier. In an example in which no modulationwas used (as is the case with direct absorption), the proces-sor 400 does not perform demodulation of the detector

60 signal and instead processes the raw data provided by thedetector 110. The processor 400 may then average thespectral data acquired from the raw/demodulated data overa period of time (e.g., one second) and compare the resultagainst reference spectral data (e.g., modeled data or spectral

65 data collected in the absence of the trace gas 112) todetermine the concentration of the trace gas 112. In so doing,the processor 400 may compare the spectral characteristics

US 9,671,332 B2I

of the processed data with those of the reference data (e.g.,the modeled data). The spectral characteristics may include,for example, peak amplitude, linewidth, and center wave-length of the averaged spectral data. Further, the processor400 may control and facilitate communication between the 5

above-mentioned constituent blocks of the electronic system102.The laser driver 404 may deliver a preset current to the

laser 104 for exciting the laser 104 and producing a light ofa preset wavelength. In an embodiment, the electronic iosystem 102 may further include a laser current modulationcircuitry to enable modulation (e.g., 2f modulation) of thelaser drive current.The processor 400 may instruct the laser driver 404 to

modulate the drive current of the laser 104, or to turn off 15modulation, depending on whether the OPLS 100 is oper-ating under direct absorption or modulation schemes. In anembodiment, the OPLS 100 may alternate (e.g., periodicallyalternate) between modulation mode and direct absorptionmode. In so doing, the OPLS 100 may utilize the measure- 20ment results under direct absorption mode to calibrate themeasurement results from modulation mode. For example,direct absorption concentration values may be interleavedwith modulation concentration values and the modulationconcentration values may be scaled to match the interpolated 25direct absorption results, which may be more accurate butless precise.

In one example, the laser driver 404 also includes athermoelectric cooler (TEC) driver 405 for operating a TEC105 inside the laser 104 to regulate its temperature and to 30properly tune the light emission source (e.g., the semicon-ductor laser) of the laser 104. Inadequate temperature con-trol may lead to changes in temperature of the light emissionsource, which, in turn, may adversely affect the spectra ofthe emitted light beam (by, e.g., shifting its center wave- 35length), and hence reduce absorption sensitivity and/ormeasurement accuracy. In one example, the processor 400monitors the temperature readings from a temperature sen-sor on (but external to) the light emission source, andinstructs the TEC driver 405 to compensate for any drifts in 40temperature.

According to an embodiment of the present invention, inaddition to, or in lieu of, monitoring the temperature at thelight emission source, the processor 400 detects temperaturechanges inside of the light emission source by monitoring 45shifts in spectra (e.g., shifts in center wavelengths of thespectra) of the detected light, which has interacted with atrace gas. Shifts beyond a predetermined noise thresholdmay prompt the processor 400 to signal the TEC 105 toadjust the temperature of the light emission source of the 50laser 104 by an amount sufficient to compensate for the shiftin spectra (e.g., shift the emission wavelength versus currentof the laser 104 back to its original position). In an embodi-ment, the emission wavelength versus current of the lasermay be determined by real-time processing of the data, 55which assesses the linecenter(s) of the trace gas resonance(s)in relation to the laser current. According to an embodiment,the temperature control feedback may be dampened by analgorithm that prevents rapid changes to the laser tempera-ture control in the event that the real-time processing is 60corrupted by data anomalies such as blockage of the light.

In an embodiment, the OPLS 100 is equipped with anumber of sensors, which may enable the OPLS 100 to besituationally aware. For example, the GPS 406 may track thegeographical location of the OPLS 100 and automatically 65synchronize the location information with any other datacollected/processed by the OPLS 100. Thus, the OPLS 100

10may be utilized to construct a spatial map of the trace gasconcentrations. Further, the ancillary sensors 408 mayinclude an integrated pressure sensor and/or a temperaturesensor for measuring ambient pressure and/or temperature ofthe atmosphere 114. The temperature and pressure readingsfrom the ancillary sensors 408 may be used by the processor400 in determining the concentration of the trace gas 112based on the data received from the detector 110.In an embodiment in which high measurement accuracy is

desired, as may be the case, for example, when using theOPLS 100 to measure concentration variations in remotegeographical locations where the changes in trace gas con-centrations may be very small (e.g., on the order of tens ofparts-per-billion over several seconds), the processor 400may rely on ambient temperature readings from a tempera-ture sensor (e.g., a thermistor) at the analysis region 106(e.g., fastened to a connecting rod holding the mirrors108/109). The temperature readings of such a sensor maymore accurately represent the temperature of the moleculesof the trace gas 112 probed by the laser light beam than thereadings of the temperature sensors of the ancillary sensors408.

Through the transceiver 410, the OPLS 100 may be ableto communicate with an external device. For example, thetransceiver 410 may include a Bluetooth transceiver capableof wirelessly transmitting geo-location and sensed data to amobile phone, a laptop, and/or a central server. Further, thetransceiver 410 may permit wired serial communicationwith the external device by utilizing, for example, therecommended standard 232 (RS-232) and/or the transistor-to-transistor logic (TTL) serial communication protocols.The I/O port 412 may enable the OPLS 100 to read/writedata onto a portable media, such as a secure digital (SD) card(e.g., a microSD disc).

Other embodiments of the electronic system 102 arewithin the scope and spirit of the present invention. Forexample, the functionality described above with respect tothe electronic system 102 can be implemented using soft-ware, hardware, firmware, hardwiring, or combinationsthereof. One or more computer processors operating inaccordance with instructions may implement the function ofthe electronic system 102 in accordance with the presentinvention as described above. It is within the scope of thepresent invention that such instructions may be stored onone or more non-transitory processor readable storage media(e.g., a magnetic disk, non-volatile random-access memory,phase-change memory or other storage medium). Addition-ally, modules implementing functions may also be physi-cally located at various positions, including being distrib-uted such that portions of functions are implemented atdifferent physical locations.FIG. 5 is a flow diagram illustrating a process 500 for

detection of a trace gas 112, according to an illustrativeembodiment of the present invention.At act 502, the open-path laser spectrometer (OPLS) 100

illustrated in FIG. 1 is provided, which includes an analysisregion 106 having first and second mirrors 108/109 at adistance D from each other, a laser 104 having a lightemission source and a thermoelectric cooler (TEC) 105 forcontrolling the temperature of the light emission source, adetector 110, and an electronic system 102. The laser 104,which is adjacent to the first mirror 108, emits a light of awavelength range (e.g., a nearly fixed linewidth) toward thesecond mirror 109. The emitted light beam may reflect off ofthe first and second mirrors 108/109 a plurality of timesbefore impinging on the detector 110, which may be adja-cent to the second mirror 109.

US 9,671,332 B211

At act 504, the analysis region 106 of the OPLS 100 isexposed to the ambient atmosphere. Accordingly, the analy-sis region 106 and the volume between the first and secondmirrors 108/109 is at (e.g., is exposed to) the ambienttemperature and pressure. 5

At act 506, the detector 110 detects a characteristic (e.g.,the intensity) of the impinging light beam and generates anelectrical signal corresponding to the detected characteristicof the impinging light.At act 508, the electronic system 102 adjusts (e.g., tunes) io

the wavelength range of the emitted light beam from thelaser 104 based on the generated signal. The electronicsystem 102 may adjust the wavelength range (e.g., shift acenter wavelength of the wavelength range) by detecting awavelength drift in the spectrum of the generated signal and 15signaling the TEC 105 to control the temperature of the lightemission source of the laser 104 to compensate for thewavelength drift. According to an embodiment, the elec-tronic system 102 adjusts (e.g., tunes) the wavelength rangeof the emitted light beam by further detecting temperature 20changes based on readings from the temperature sensor on(but external to) the light emission source of the laser 104.

At act 510, a concentration of the trace gas 112 ismeasured based on the generated signal. The electronicsystem 102 calculates one or more spectral characteristics of 25the generated signal, such as peak amplitude, linewidth, andcenter wavelength, and compares the result(s) against ref-erence data (e.g., modeled data), to determine the concen-tration of the trace gas 112.The present invention is not to be limited in scope by the 30

specific embodiments described herein. Indeed, other vari-ous embodiments of and modifications to the present inven-tion, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoingdescription and accompanying drawings. Thus, such other 35embodiments and modifications are intended to fall withinthe scope of the present invention. Further, although thepresent invention has been described herein in the context ofa particular implementation in a particular environment fora particular purpose, those of ordinary skill in the art will 40recognize that its usefulness is not limited thereto and thatthe present invention may be beneficially implemented inany number of environments for any number of purposes.Accordingly, the claims set forth below should be construedin view of the full breadth and spirit of the present invention 45as described herein.

What is claimed is:1. An open-path laser spectrometer for measuring a con-

centration of a trace gas, the open-path laser spectrometer 50comprising:

an open-path multi-pass analysis region comprising a firstmirror, a second mirror at a distance and orientationfrom the first mirror, and a support structure for locat-ing the first and second mirrors; 55

a laser coupled to the open-path multi-pass analysisregion and configured to emit light of a wavelengthrange and to enable a plurality of reflections of theemitted light between the first and second mirrors;

a detector coupled to the open-path multi-pass analysis 60region and configured to detect a portion of the emittedlight interacting with the trace gas and impinging on thedetector and to generate a return signal correspondingto a characteristic of the detected portion of the emittedlight; and 65

an electronic system coupled to the laser and the detector,wherein the electronic system:

12monitors a spectrum of the generated return signal to

determine a shift in a resonance line-center of thetrace gas with respect to an injected laser current, ascompared to an initial resonance line-center of thetrace gas with respect to the injected laser current;

adjusts a temperature of the laser, based on the deter-mined shift, to return the resonance line-center of thetrace gas with respect to the injected laser currentback to the initial resonance line-center of the tracegas with respect to the injected laser current; and

measures the concentration of the trace gas based onthe generated return signal.

2. The open-path laser spectrometer of claim 1, whereinthe trace gas is methane.

3. The open-path laser spectrometer of claim 1,wherein the trace gas exhibits a resonant frequency

response in the wavelength range of the emitted light,and

wherein other gases in the atmosphere do not exhibitresonant frequency responses in the wavelength range.

4. The open-path laser spectrometer of claim 1, whereinthe laser comprises a semiconductor laser diode operating inan infrared range.

5. The open-path laser spectrometer of claim 1, whereinthe wavelength range is from about 3.2 µm to about 3.5 µm.

6. The open-path laser spectrometer of claim 1, whereinthe open-path multi-pass analysis region is exposed to theambient atmosphere.

7. The open-path laser spectrometer of claim 1, whereinan optical path in the open-path multi-pass analysis region isat near ambient temperature and at near ambient pressure.

8. The open-path laser spectrometer of claim 1,wherein the open-path multi-pass analysis region com-

prises a Herriott cell,wherein the first and second mirrors are opposing concave

mirrors, andwherein the first mirror comprises a first hole configured

to allow light to enter and/or exit the open-path multi-pass analysis region.

9. The open-path laser spectrometer of claim 8,wherein the first hole is configured to allow the emitted

light to enter the open-path multi-pass analysis region,and

wherein the second mirror comprises a second hole con-figured to allow the reflected light to exit the open-pathmulti-pass analysis region.

10. The open-path laser spectrometer of claim 1, whereinthe open-path laser spectrometer is configured to utilize adirect laser absorption and/or 2f modulation/demodulationspectrometry techniques.

11. The open-path laser spectrometer of claim 1, whereinthe electronic system comprises a global positioning system(GPS) configured to track a location of the open-path laserspectrometer and to synchronize the location with the gen-erated return signal of the detector.12. The open-path laser spectrometer of claim 1, wherein

the electronic system further comprises a wireless trans-ceiver configured to enable communication between theopen-path laser spectrometer and an external device.13. The open-path laser spectrometer of claim 1, wherein

the open-path laser spectrometer has a trace-gas detectionsensitivity of about 10 parts per billion (ppb) in 1 second.14. The open-path laser spectrometer of claim 1, wherein

the open-path laser spectrometer is portable and hand-held.15. The open-path laser spectrometer of claim 1, wherein

the distance between the first and second mirrors is between

US 9,671,332 B2

13about 8 cm and about 20 cm and a total optical path lengthof the open-path multi-pass analysis region is more than 4 in.

16. The open-path laser spectrometer of claim 1, furthercomprising a thermoelectric cooler (TEC) configured tocontrol a temperature of an emission source of the laser,

wherein the electronic system is configured to adjust thetemperature of the laser by signaling the TEC to controlthe temperature of the emission source of the laser tocompensate for the determined shift in the resonanceline-center of the trace gas with respect to the injectedlaser current.

17. A method for measuring a concentration of a trace gascomprising:

providing an open-path laser spectrometer comprising:an analysis region comprising a first mirror and a

second mirror at a distance from the first mirror;a laser;a detector; andan electronic system,wherein the laser is configured to emit light of a

wavelength range toward the second mirror, theemitted light reflecting off of the first and secondmirrors a plurality of times before impinging on thedetector;

exposing the analysis region of the open-path laser spec-trometer to the ambient atmosphere;

detecting, by the detector, a characteristic of a portion ofthe emitted light interacting with the trace gas andimpinging on the detector, and generating, by thedetector, a return signal corresponding to the detectedcharacteristic of the impinging light;

monitoring, by the electronic system, a spectrum of thegenerated return signal to determine a shift in a reso-

14nance line-center of the trace gas with respect to aninjected laser current, as compared to an initial reso-nance line-center of the trace gas with respect to theinjected laser current;

s adjusting, by the electronic system, a temperature of thelaser, based on the determined shift, to return theresonance line-center of the trace gas with respect tothe injected laser current back to the initial resonanceline-center of the trace gas with respect to the injected

10 laser current; andmeasuring, by the electronic system, a concentration of

the trace gas based on the generated return signal.18. The method of claim 17, wherein exposing the analy-

sis region comprises exposing an optical path in a region15 between the first and second mirrors to an ambient tempera-

ture and an ambient pressure.19. The method of claim 17,wherein the trace gas exhibits a resonant frequency

response in the wavelength range of the emitted light,20 and

wherein other gases in the atmosphere do not exhibitresonant frequency responses in the wavelength range.

20. The method of claim 17, wherein the open-path laserspectrometer further comprises a thermoelectric cooler

25 (TEC) configured to control a temperature of an emissionsource of the laser,

wherein adjusting the temperature of the laser comprises:signaling, by the electronic system, the TEC to control

the temperature of the emission source of the laser to30 compensate for the determined shift in the resonance

line-center of the trace gas with respect to theinjected laser current.